Our long-range goal, simply stated, is to determine how gross structure, fiber type composition and whole-muscle mechanics affect recruitment during various movements. Specifically, we have planned a series of projects to study the recruitment of synergistic skeletal muscles, classified primarily as fast or slow, required to produce forelimb and hind limb movements of varying velocity and force. To this end, the kinetics and kinematics of selected joints (ankle, knee, elbow) will be described for locomotion, jumping, landing and novel non-supportive movements, and recruitment patterns during these movements and the dynamic properties of selected slow and fast synergists of these three joints will be compared. Also, we plan to study recruitment of slow and fast muscles during peripheral and central perturbations of fatigue, muscle isolation, joint immobilization and spinalization. It is anticipated that from these studies our knowledge of the role of fast and slow muscles, which differ dramatically in their metabolic and mechanical properties, will be increased, and that we will gain further understanding of the selective recruitment of motor units to affect efficient and effective movements under normal conditions.